Aircraft: FIGHTER
Phase of flight: Cruise
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy

I opened/split out Sector 36/68 to help with frequency congestion. Earlier; Aircraft Y was cleared into IR126; but I wasn't made aware of it. I was made aware of Aircraft X and was given the exit time and requested altitude to coordinate with ZLA7. When I called ZLA7 to coordinate Aircraft X's exit time; they alerted me to Aircraft Y and its exit time; and the possible conflict. I had no track or information on my scope or EDST about Aircraft Y and verified they weren't mistaken on the time. I found out subsequently that Aircraft Y was cleared into IR126 a couple of controllers back; and the data block was dropped when ZLA7 did not take the handoff/transfer of the data block. While speaking to ZLA7 I asked of the known or estimated position of Aircraft Y. They didn't know and had to assume it was an overdue aircraft. Shortly thereafter Aircraft Y's beacon showed up in ZLA7's airspace right about the time and place it should've been; which was approximately 25 miles in front of Aircraft X's estimated position. I attempted to contact Aircraft X on all of my available frequencies to no avail. ZLA7 got ahold of Aircraft Y and climbed them above the IR126 altitudes. In the 19 years I've worked this airspace; and the countless times I've worked aircraft into IR126; I've never seen this occur. Turns out it was a possible pilot deviation (and being treated as such) because the scheduling agency made Aircraft X aware of Aircraft Y and the delay necessary to properly separate; only to have Aircraft X apparently circumvent that delay by entering at an alternate entry point further downstream along IR126. With the combining; changing of controllers and then de-combining of 36 (from 12); it might've added to the confusion. While exit time was coordinated; the data block of Aircraft Y's position was dropped; which is not uncommon with this procedure. I would suggest; and have always practiced; that the data block remain; be put in COAST status so it follows along with where the aircraft should be. I've found that even though it's a very long time with no radar; when the aircraft finally appears on radar (usually at the ZLA/ZDV boundary) the COAST track is very close to the actual aircraft. Somehow Aircraft Y's info fell out of the EDST; either by the previous controller or in the opening of sector 36. Had it been there; especially with the exit time in the scratchpad (which is a very common practice) the controller who cleared Aircraft X would've questioned the timing and if MARSA (Military Assumes Responsibility for Separation) was possible. We assume; rightly; that the scheduling authority will not put two aircraft into an IR route in conflict like this; which they did attempt; only to be overridden by the pilots; apparently. While the pilot is not required to monitor our frequency; they usually monitor guard. We used to have the ability to bring up VHF guard from Abajo Peak which has a possibility of reaching aircraft at low level in IR126. That ability to select it from the sector has been taken away; it probably shouldn't have. VHF guard would not have worked with Aircraft X possibly; but it would've with Aircraft Y. As I was accepting the sector from the controller I was relieving; I was aware of Aircraft Y in IR-126. The relieving controller said he would pass the exit estimate to LA Center before leaving. I tracked the flight after they entered the IR route until the target dropped from radar. I initiated a coast track which worked for a while; but then it started free tracking. I moved the data block close to the ZLA boundary and initiated a handoff; which they ignored. After a while; I dropped the data block and continued working.A while later I received a handoff on Aircraft X from Albuquerque Center going into IR-126A. I cleared the aircraft into IR-126A after receiving the exit estimate and requested altitude upon exit. At that moment my sector was getting split off and I verbally coordinated with the receiving controller the estimates that he would need to pass to LA Center. When he passed the information; LA center asked about Aircraft Y. It is my understanding that the military schedules the entrance and exits of their military aircraft in the IR routes. Since the entrance is in one Center and the exit is in another Center; how would we ever know it the preceding aircraft has exited or not? ZDV controllers report of an unsafe situation where they uncertain where two aircraft were since one aircraft was in the COAST mode. Controllers could not find the aircraft until it exited restricted airspace.